Because of the dramatic drop in cost of computer memory, it is becoming more economical and efficient to store film data in computer archival storage than in the film media itself. For example, in a large hospital hundreds of X-ray images are made every day. In the past these X-ray films were often stored indefinitely. It has been recognized that computer storage of such films is preferable if an image can be read and reconstructed with accuracy. Computer storage of such images is accomplished by laser writing and reading systems.
Laser writing and reading systems are known. For example, see U.S. Pat. No. 4,225,224 to N. Balasubramanian. Such systems commonly use photosensitive media for pre-recorded data being read or for writing new data. In either case, it is typical that a laser beam must rapidly scan the media, which is either reflective or transmissive of laser light.
The laser beam usually scans the medium by means of scanning optics, typically a scanning mirror or a rotating polygonal wheel having reflective surfaces. Such scanning is along a line extending across the width of the medium. To attain areawise media coverage, the medium is advanced relative to the beam, preferably in synchronism. Once a line has been scanned, the medium, usually film, is incrementally advanced a slight distance and an adjacent line is scanned. This is usually accomplished by mounting the film on a drum and rotating the drum by a slight amount after each line scan. Rotation is carried out by a stepper or other type of motor until the entire film has been scanned.
A problem with this approach is that the motor introduces a slight amount of vibration to the film, no matter how smooth the motor may be. This vibration causes film jitter which is seen on the film as lines, sometimes interfering with the ability to read data. In all cases it is easier to read data when there is no mechanical vibration of the film.
To solve this problem, prior application Ser. No. 551,266, now U.S. Pat. No. 4,505,578 disclosed a method and apparatus of providing relative motion between a scanning beam and a photosensitive medium which is virtually free from vibration. Rather than advancing film by means of motors or the like, a transport relies on gravity for providing a vibration-free force to obtain relative motion. Yet gravity alone is insufficient because gravity provides acceleration. For film scanning, a uniform velocity is required so that the scan position can be accurately known. A hydraulic brake is used to place drag on falling mass associated with a carriage carrying either the photosensitive medium or the read/write optical system.
A mass associated with a carriage is supported over a hydraulic cylinder and piston, the cylinder containing hydraulic fluid. The mass falls downwardly in the earth's gravitational field, working against the piston. A valve is connected to the hydraulic cylinder metering the flow of hydraulic fluid from the cylinder due to work done by the piston. The rate of fluid flow is such that the valve acts as a brake limiting the rate of fall of the piston and hence the carriage. If the read/write optical system is mounted on the carriage, the photosensitive medium is mounted in a fixed position in optical communication therewith, or vice versa.
The falling mass and hence the carriage is free from detectable vibration as measured on the photosensitive medium. While formerly vibration manifested itself in the form of faint lines streaking the medium resembling a raster effect, the present invention shows no similar lines and enhances the signal-to-noise ratio of laser written and read data.
A problem which is encountered is that wear on mechanical components of the transport, temperature differences in the hydraulic fluid passing through the valve and perhaps other effects cause small velocity variations in the transport. Such variations mean data inaccuracies in reading or recording data, since motion of the transport is assumed to be uniform.
An object of the invention was to devise a means for offsetting small perturbations causing velocity non-uniformity in the transport.